Oxygen is life, but in the case of foods, it is not so. When harvested, fresh fruits and vegetables are at the peak of their quality. Their quality cannot be improved; it can only be deteriorated or maintained.
Fresh, high-quality products are the primary requirements for the national and international food industry during this current era. After harvesting, especially for fresh fruits and vegetables, continue their respiration process. The respiration rate has to be reduced to maintain quality, especially when the products are stored for an extended period or shipped to distant markets. The best way to preserve quality and extend shelf life is by cooling, and another method used to extend shelf life is the modification of the atmosphere surrounding the product.
Also known as “CA storage” in the produce business, Controlled atmospheric storage is the storage in which external control systems control the atmosphere of oxygen, carbon dioxide, and nitrogen (and sometimes other gases).
Outside air consists of approximately 78% nitrogen (N2), 21% oxygen (O2), 0.045% carbon dioxide (CO2). CA lowers the oxygen level generally to 0.5-2.5%, depending on the type of product and the variety. CA conditions extend the shelf life of fruit and vegetables by a factor of 2 to 4.
The principle behind controlled and modified atmosphere technologies is to reduce-
(a) respiration rate
(b) reduce microbial growth
(c) retard enzymatic spoilage by changing the gaseous environment surrounding the food product. This is achieved by reducing the concentration of oxygen (O2), which is required in respiration, or by adding an inhibitory gas such as carbon dioxide (CO2) or carbon monoxide (CO). The balance between O2 and CO2 is critical, and an optimal ratio is required for each specific product.
The CA implies a higher degree of control than MA in maintaining specific levels of O2, CO2, and other gases.
Requirements for Ideal CAS :
- CA Storage Room: A gas-tight room is an obvious prerequisite for achieving a well-controlled atmosphere storage system. Thus it is necessary to make room walls gas-tight. To ensure that the walls are gas-tight to CA storage, they are lined with sheets of galvanized steel.
- Temperature Control: CA storage is only successful when applied at low temperatures. Standard refrigeration units are, therefore integral components of the CAS system. Temperature control is achieved by having pipes containing a refrigerant inside the storage. Ammonia or chlorofluorocarbons are common refrigerants.
- Humidity Control: Most fruits, vegetables, and meats kept in CA storage, require high relative humidity, generally the closer to saturation the better, so long as moisture does not condense on the foods.
- Gas Control: The atmosphere in a modern CA store is continuously analyzed for CO2 and O2 levels using an Infrared gas analyzer to measure the gas content in the store regularly. There are also ethylene analyzers that continuously measure ethylene concentration in the store. In storage rooms where low ethylene is essential, checks can be made to operate the ventilation and ethylene removal systems correctly.
- Scrubbers: The composition of the gas mixture inside the storage rooms undergoes continuous change as a function of the stored product’s metabolic activity and scrubbers are necessary to absorb excess CO2. Scrubbers are generally classified according to the absorbent material: Ca(OH)2, NaOH, H2 O, zeolites, activated charcoals.
Methods of CA storage :
- Scrubbers :
To regulate CO2 levels in a CA room, one of the simplest methods is the hydrated lime (Calcium carbonate) Ca(OH) 2 “scrubber”. The scrubber is constructed using an insulated and airtight plywood box located outside the CA room. The box should contain enough lime for the entire storage period, but new lime can be added if CO2 absorption drops.
The CO2 and hydrated lime react in a 1:1 ratio to form limestone and water. Lime with a smaller particle size (fine mesh) is more efficient at reacting with CO2 than lime with a coarse mesh.
- Using plastic tents:
Controlled atmosphere storage of pallet loads of produce is also possible using a semi-permanent set-up for creating a gas seal. Any number of pallets can be accommodated inside a plastic tent made using. The seal is then made by joining the tent plastic with the floor plastic (using tape).
Benefits of CAS
In fresh fruits and vegetables stored under optimal CA or MA, practical quality advantages include:
- Reduction in chlorophyll breakdown, resulting in higher color stability.
- Reduction in enzymatic browning in cut produce, whenever low levels of O2 are used.
- Improvement in texture caused by the action of CO2 on enzymes acting on cellular membranes.
- Reduction in some physiological disorders induced by C2H4, such as scald of apples and pears and chilling injury of citrus fruits, avocado, chili pepper, and okra.
- Reduction in microbial activity, especially molds.
World trade has transformed food retailing and driven the development of technology to transport and store horticultural products, providing a year-round supply of fruit and vegetables. Horticultural produce is highly perishable, as fruit and vegetables continue their metabolic processes, leading to ripening and senescence after harvest, making them ultimately unmarketable. Advanced postharvest technologies are essential for reducing food waste while maintaining high standards of safety and quality. Thus cold storage together with, controlled atmosphere (CA) and modified atmosphere packaging (MAP) have been applied to alter the product’s internal and external environment, decreasing its metabolic activity and extending shelf-life. Both CA and MAP have benefitted from technological innovation. It is important to note that Controlled atmospheric storage and modified atmospheric storage should be used as a supplement and not as a substitute for proper temperature and relative humidity.